1. FIELD OF THE INVENTION
The present generally relates to semiconductor lasers and, in particular, to a broad area surface emitting distributed feedback semiconductor laser that incorporates a curved pattern in a second order grating to produce a predominantly single-lobed lateral mode far-field output intensity profile at relatively high power.
2. DESCRIPTION OF THE PRIOR ART
Light amplification by the stimulated emission of radiation (laser) produces unidirectional, monochromatic, and, most importantly, coherent visible light. The stimulated emission of radiation is a process in which the energy state of an atom changes in a quantum transition with the emission of a photon. During such a process, a photon approaches an atom, initially in an excited energy state, and induces this atom to make a transition to a lower energy state. As the atom's energy state is lowered, the atom emits a photon. This emitted photon, which is separate from the photon that induced the energy transition, possesses an energy that is equal to the difference between the excited and the lower energy states of the atom. Moreover, this emitted photon and the inducing photon both leave the atom in the same direction the inducing photon had as it approached the atom. These exiting photons are also exactly in relative phase with one another; that is, they are coherent. This coherence is dictated by energy conservation in that if the two photons were out of phase by any amount they would interfere destructively, thereby violating energy conservation. Therefore, stimulated emission of radiation is a process that induces coherent photon multiplication or light amplification, thus a laser.
Laser technology has evolved by applying the above stated principle to several different types of active media. The most recent development in this field, coupled with the advancements in semiconductor fabrication technology, is the semiconductor laser. Unlike prior lasers, however, stimulated emission in a semiconductor laser occurs when there is an excited state of a solid state material. Thus, stimulated emission in a semiconductor laser involves more than one atom.
A surface emitting distributed feedback semiconductor laser is a device that produces unidirectional, monochromatic, coherent visible light through stimulated emission in semiconductor materials. Such a device has a positively doped side and a negatively doped side that are joined at a junction, and a grating that is etched into an outer surface of the positively doped side. The surface of the grating, upon which a strong conductive material is disposed, provides a means by which coherent photon energy fields may be diffracted. A second order grating design permits deflections of coherent photon radiation to be directed normal to an output window etched into the negatively doped side of the junction through first order diffraction, and directed parallel to the surface of the grating through second order diffraction. The first order diffraction produces a beam of unidirectional, monochromatic, coherent visible light at the output window, whereas the second order diffraction provides a feedback of photon radiation to an active region that is adjacent and parallel to the surface of the grating. Much has been written on the subject of semiconductor lasers in recent years and some good descriptive background articles on these devices are Surface Emitting Distributed Feedback Semiconductor Laser, Applied Physics Letters, Volume 51, Number 7, pp. 472-474, August 1987, and Analysis of Grating Surface Emitting Lasers, IEEE Journal of Quantum Electronics, Volume 26, Number 3, pp. 456-466, March 1990.
Along the length of a surface emitting distributed feedback semiconductor laser diode device having a uniform, or linear, grating, a theoretical longitudinal mode near-field output intensity profile is double-lobed and antisymmetric with a zero intensity null at the output window center. A corresponding theoretical longitudinal mode far-field output intensity profile is double-lobed and symmetric about the output window center. These theoretical longitudinal mode output intensity profiles have been substantiated in actual device measurements, although in these measurements it has been found that spontaneous emission partially fills the near-field intensity null at the output window center. Nonetheless, the longitudinal mode output intensity profiles associated with a surface emitting distributed feedback semiconductor laser diode device having a linear grating are acceptable for many applications due to a consistent mode relationship between the first and the second order diffracted photon radiation along the length of the grating.
Along the width of a surface emitting distributed feedback semiconductor laser diode device having a uniform, or linear, grating surface, however, a lateral mode near-field output intensity profile and a lateral mode far-field output intensity profile are not acceptable for many applications due essentially to self-guiding and filamentation effects. Such effects result in the lateral mode near-field output intensity profile having an increasing number of lobes as the width of the linear grating increases. Furthermore, there is a 180 degree near-field phase shift between each near-field lobe. The corresponding lateral mode far-field output intensity profile displays a double-lobed pattern centered about the location of each 180 degree near-field phase shift.
One reason that the lateral mode output intensity profiles associated with a surface emitting distributed feedback semiconductor laser diode device having a linear grating are not acceptable for many applications is that it is often required that the width of a grating be increased, or a broad area grating be created, in order to increase the output beam power of the device. Such an increase in grating width, although allowing an increase in the power applied to the device, results in an increase in the number of far-field lobes, which is unacceptable for many applications. It is therefore desirable to increase the output beam power of a broad area surface emitting distributed feedback semiconductor laser diode device, while concentrating that output beam power into a single far-field lobe.